According to Chao(1965):
A form in the widest sense is any fraction of a language, be it a word or any succession of words. In practice, however, the smallest units, such as phonemes, features of a phoneme, say voicelessness, or very large units, such as a whole lecture or a play, are not forms as object of grammatical analysis. Only the smallest unit that has meaning, the morpheme, and the largest unit between pauses, the sentence, and units of intermediate sizes have generally been included in grammatical analysis. Forms usually occur in succession, except for a relatively small number of simultaneous or overlapping forms, such as an interrogative intonation over a whole sentence, which can be considered a morpheme having a meaning but residing parasitically on the other forms, the words in the sentence. Forms which are worth considering in grammar have a certain unity such that they will recur with similar forms in similar contexts. Such forms are then constituents, as distinguished from any fraction of a larger form, cut out arbitrarily, which makes up a string but not necessarily a constituent. For example, in 说中国话 shuo Jong. gwohuah ‘speak Chinese’, each of the syllables, which is also a morpheme, is a constituent. Furthermore, shuo, Jong. gwohuah, Jong. gwo, and huah are constituents, but shuo Jong and . gwohuah are not constituents. In other contexts shuo Jong. gwo ‘speaks of China’ could be a constituent, but it is not one in shuo Jong. gwohuah. It goes without saying that a fractional part of a morpheme plus whatever follows or precedes will make up a string but not a constituent.
 It is an awkward discrepancy in usage between Chinese and Western writers that Chinese 形 shyng ‘form (of characters)’ is contrasted with 音 in ‘sound’ and 义 yih ‘meaning’, while ‘form’ in Western usage corresponds really to in and not to shyng. The confusion is compounded when recent Chinese writers translate ‘morphology’ as 形态 shyngtay ‘form-state’ and when recent Western writers go further and use ‘shape’ for a particular phonemic make-up of a form. However, one can usually tell what an author means, given the context of use.
A beginning, however, has been made in the formal study of larger units by Zellig S. Harris, in his “Discourse Analysis”, Language, 28:1-30, and “Discourse Analysis: A Sample Text”, Language, 28： 474-494 (1952).
Chao Y R. A grammar of spoken Chinese[M]. Univ of California Press, 1965.